In the paper pulp industry, and more particularly in the industry of production of paper pulp from used paper, a large number of sieves is used both for separating the fibers forming the paper pulp from the various impurities (called "contaminants") occurring in used paper, in equipment called "strainers", as well as for sorting the fibers according to their length in equipment called "classifiers".
It is known how to make such sieves by providing them with holes or slots, and it is also known from numerous patents such as FR 1,539,846; U. S. Pat. No. 3,617,008; SE 72/11272; FR 78 08132 and FR 88 10684 how to arrange, upstream of the slots or holes made through the wall of the sieve, bar ("obstacles") followed by grooves which, in cooperation with a hydrodynamic fan, cause pulsations which improve the efficiency of the sieve and prevent it from becoming clogged.
But these sieves, whether they have holes or slots and whether or not they are provided with bars (obstacles), have until now been made by machining solid plates, with great difficulty.
In fact, the slots and holes must, for reasons of loss of charge and fouling, be very short, that is, of the order of 0.5 to 1 mm; the plates used in modern technology, however, are must thicker, of the order of 8 to 10 mm, for reasons of strength, and as the performances demanded by the users keep increasing, the thicknesses of the plates increase. As a result, it is necessary to machine a clearance in the thickness of the plate and then to clean and polishthis clearance, these two operations representing the most important work that is carried out on the plates.